Non-Foster circuit elements, such as negative capacitors and inductors, have been studied for many years as a means to extend the bandwidth of antennas to the low frequency (i.e., small antenna) regime. An example is: G. Skahill, R. M. Rudish, and J. A. Piero, Apparatus and method for broadband matching of electrically small antennas, U.S. Pat. No. 6,121,940.
There have been some publications that report simple inter-element coupling using non-Foster elements, such as: R. C. Hansen, “Wideband Dipole Arrays using wideband coupling,” IEEE Microwave and optical tech letters, vol. 38, no. 6, 2003. In this paper Hansen simulates a planar array of dipole elements whose ends are connected with lumped negative inductors. The simulated results show high array gain over a broad bandwidth. This approach does not attempt to decouple and impedance match the antenna modes.
Mode decoupling networks have been reported in the literature as a means to improve the performance of antenna arrays when the inter-element spacing drops below half a wavelength. Examples of decoupling networks may be found in: C. Volmer, et. al, “Broadband decoupling and matching of a superdirective two port antenna array,” IEEE antennas and propagation letters, vol 7, 2008; and J. C. Coetzee, et. al., “port decoupling for small arrays by means of an eigenmode feed network,” IEEE trans. On antennas and propagation, vol 56, no. 6 June 2008. These papers do not consider Non-Foster circuit elements to extend the performance bandwidth.